Directional antenna system



v J y 10, 9 L. RUSSELL I 3,044,063

DIRECTIONAL ANTENNA SYSTEM Filed March 19, 1959 3 Sheets-Sheet 1 FIG.|

T0 RECEIVER OR TRANSMITTER FIG.4

' INVENTOR.

LINDSAY RUSSELL v July 10, 1962 L. RUSSELL 3,

DIRECTIONAL ANTENNA SYSTEM Filed March 19, 195 9 INVENTOR.

LINDSAY RUSSELL 3 Sheets-Sheet 2 July 10, 1962 Filed March 19, 1959 a L. RUSSELL 3,044,063

DIRECTIONAL ANTENNA SYSTEM 3 Sheets-Sheet 3 L 48 (if 44 TO RECEIVER OR- TRANSMITTER INVENTOR.

LINDSAY RUSSELL United States Patent-O 3,044,063 DCTIONAL ANTENNA SYTEM Lindsay Russell, Cambridge, Mass, assignor, by mesne assignments, to Andrew Alford, Boston, Mass. Filed Mar..19, 1959, Ser. No. 800,597

Claims. ((11.343-100) eating over great distances, particularly when an interfering signal originates from a direction diiferent from that of the desired signal.

' The frequency range for long distance communication generally extends from three to thirty-six megacycles. Propagation over medium and long distances in this frequency range is obtained by refraction of high frequency waves from the E and F layers of the ionosphere. The

most efiective frequency of communication depends on the distance, the time of day, and the direction of transmission; that is, north to south, east to west, or by great circle paths over polar regions.

Polarization of the transmitted signal is believed to be of minor importance insofar as the effect of refraction of high frequency Waves from the ionosphere is concerned. The efiect of the ionosphere is such that regardless of the transmitted polarization, the polarization "at the receiving site is both vertically and horizontally polarized in about equal proportions.

r Barring a poor choice of frequency for the transmission path, ionospheric disturbances and the like, the inability to establish reliable communication is likely to be due to masking of the desired signal by atmospheric noise, unintentional interference from 'co-channel stations, or by deliberate jamming.

By using a directional antenna system, the ratio of signal to interference may be appreciably increased. For receiving antennas, the absolute gain of the antenna; that is the amount of energy it can abstract from passing radio waves, is not, within reasonable limits, a very important consideration. Stated in other words, the important factor is the pattern or directivity gain rather than'the physical efiiciency. Reducing the efficiency of a receiving antenna will not lower its eifectiveness until the point is reached where the internal noise of the receiverbegins to obscure the desired signal. t

"It is evident that azimuthal directivity is beneficial, and up to a certain point, the more the better. 'lf-the azimuth pattern gain is 12 db (relative to a circular pattern), the antenna can provide a 12 db improvement, on the average, in sig'nal-to-interference ratio. If the interference is in the form of noise arriving with equal intensity'from all directions, the improvement should be just -12 db. If the interference is due to another station, the improvement may be as much as or more db, depending} on the azimuthal separation of the wanted and unwanted'sign-als.

Diversity effect, caused by the arriving of signals in a form that is not a coherent plane wave, sets an upper limit on the azimuthal gain that it is wise to try to achieve in a high frequency ground antenna which is to make use of normal E and F layer propagation modes. 'Gnjthe basis of the present knowledge, it is believed that increases in horizontal aperture beyond the range of 400- 1000 feet will not necessarily increase the strength of received signals.

The question of best elevation pattern is somewhat more complex. The signals under consideration can be expected to arrive for the most part at elevations besome Patented July 13,- 1962 tween 3 and 33 degrees. Unless elevation steering is employed, the elevation pattern is preferably broad enough to cover this range adequately. 'Itis possible that signals coming from stations within 500 miles via the F layer may arrive at higher angles. The rejection of man made interference signals by means of elevation directivity does not appear to be very effective in most practical applications. In fact, the only advantage in making the antenna non-responsive to signals arriving at angles higher than 30 will be the possible elimination of interference originating within a range of about 5% miles and arriving at a high angle at a time when the desired signal is arriving at a low angle. 1

Too much elevation directivity is probably worse than too little for long distance communication with moving,

stations. White antennas with narrow elevation patterns may sometimes be desirable in fixed path ground-toground communication, the variables of azimuth direction, range, time of day, year, and other factors make it more likely that high elevation directivity would impair the ability of a fixed ground station to communicate with a mobile station. -In the case of a transmitting antenna, elevation pattern gain would be as variable as azimuth pattern gain.

Azimuth steerability is important for establishing communication with a mobile station since the latter may be in any' azimuth direction from the receiving station. While some directions are more often used than others, it is not unlikely that many ground stations must establish communication with mobile stations in nearly every direction at some time. This is especially true if the installation is to be used in connection with direction finding.

steerable antenna systems are'known in the art, some being continuously steerable while in other systems the major lobe of the directivity pattern is changed in stepwise increments. Continuous steering is especially advantageous where a signal from another station obscures the desired signal and the operator, by monitoring the receiver output can makeadjustrnents to reorient the antenna pattern to minimize the interference. With a continuously steerable antenna system, it is sometimes possible to use nulls in the pattern to good advantage in rejecting an interfering signal. Backscatter sounding and direction finding are two other applications wherein continuous steering is especially advantageous.

On the other hand, Where a precise orientation of the directivity pattern is not required, rotation of the antenna pattern in small steps may be as satisfactory as continuous rotation. This is likely to be the case in a system employing automatic means for setting the. antennav azimuth hearing. If step azimuth pattern control is used, some directions Will be midway between adjacent steps and the full gain of the antenna will. not be fully realized.

7 High gain steerable antenna systems usable in any azimuth direction may be conveniently classified'into three categories. by itself to provide coverage in a particular direction.

7 In the second, more than one radiating element, but less than all, are used to cover a prescribed direction. In the third, all radiating elements are used simultaneously to produce a direetivity pattern with the major lobe oriented in the desired direction.

An example of a system of the first type is a circle of rhombics. it must have a high directivity of its own to satisfy the directional requirement. This first type of systemhas a number of advantages.

tions since each antenna delivers a received signal on a separate transmission line. This is especially advanta-, geous in costal marine radio stations, for example, where a number of receivers may be operated simultaneously In the first, each radiating element is used Since each rhombic is an independent unit,

A number of circuits can operate simultaneously without any particular complicato receive signals originating from different directions. Moreover, since the required azimuthal coverage of such coastal stations is generally less than 360 degrees, only a fraction of the rhombic circle need be constructed.

However, such antenna systems have a number of disadvantages. High gain rhombic antennas require a large amount of space. Moreover, these antennas provide the desired directivity over a limited band width so that a large number must be constructed in order to provide a wide band, wide angle coverage. Furthermore, continuous scanning in the azimuth direction is difficult because steerability is accomplished by switching from one rhombic antenna to the other.

The second type of system is exemplified by an array of monopoles disposed upon a circle concentric about a cylindrical reflecting screen. This configuration is frequently referred to as a Wullenweber after the wellknown German direction finding system installed in Denmark during World War II. Each monopole is moderately directional because of the screen behind it and only a group of consecutively arranged elements are utilized to provide a directivity pattern with the major lobe oriented in a given direction. For example, the group might consist of the individual monopoles arranged on an arc subtending one quadrant of the circle. This type of antenna system is advantageous because it is less expensive than the first type for equivalent performance and requires less space. In addition, continuous scanning may be employed. However, in its simplest form, it has a number of shortcomings. It is difficult to obtain good performance over a bandwidth in excess of one octave because in conventional systems, the individual radiator should be approximately one-quarter wavelength from the reflecting screen. If too close, feeding is difficult because of impedance mismatch. If too far away, they do not radiate radially away from the screen, and individual patterns develop a null in the desired forward direction.

An example of the third type is disclosed in the copending application of Lindsay Russell entitled Steerable Antenna Array, Serial Number 451,754, filed August 24, 1954, and now Patent Number 2,968,808, granted January 17, 1961. Vertical monopoles arranged on a circle are energized simultaneously in a co-phasal feed plane to produce a directivity pattern with the major lobe oriented in a desired direction. This third typeof system has a number of advantages. It is broad band since the individual radiators are each omnidirectional regardless of frequency, and phasing, based on time delays, is correct at all frequencies. Moreover, from the standpoint of economy and required space for equivalent gain, it is superior to the other types because all elements are employed to produce a pattern, whereas with the other types only a fraction of the structure contributes at any one time. The primary disadvantage of this third type of system is that the directivity patterns have minor lobes of significant amplitude. Some minor lobes are only 5.5 db below the major lobe at some frequencies within the normal operating band.

The present invention contemplates and has as an important object, the provision of an antenna system which incorporates the best features of all three types of steerable antenna systems discussed above. More specifical- 1y, it is an object of this invention to provide an antenna system characterized by the clean directivity patterns of the second type of system; that is, minor lobes of low amplitude, the broad bandwidths of the third type of system; and the multiple beam capabilities of the first type of system.

According to the invention, this is accomplished by utilizing an array of radiating elements, such as monopoles, disposed upon a curve spaced from a reflecting screen in corporation with suitable signal combining means. The combining means is normally arranged to exchange energy with a number of consecutively arranged radiating elements facing the direction in which it is desired to orient the main lobe. It may further include means for furnishing appropriate time delays to energy exchanged with the engaged group of individual radiating elements so that these elements effectively radiate or receive in phase a plane wave front traveling along the line of direction of the major lobe.

According to another feature of the invention, means are included for attenuating the amplitudes of signals exchanged with the individual radiating elements in a manner which reduces the amplitude of the minor lobes. Another advantage of the invention is that energy is exchanged at an impedance level suitable for use with an associated receiver or transmitter.

For use as a receiving antenna, a number of combiners may be utilized with the antenna system by employing isolation amplifiers. These isolation amplifiers may be, for example, cathode followers. By employing one isolation amplifier in each radiator feedpath, the radiator may contribute to a number of combiners, instead of just one, thereby providing independent multiple beam operation.

Still another advantage of using isolation amplifiers is that the frequency range of the system is extended to provide coverage over a range of six octaves. This wide band coverage may be accomplished if the amplifiers are arranged to have input and output impedances which closely match the associated transmission lines. As a result, degeneration of the impedance match at the individual radiators does not result in large phase and amplitude errors in the high frequency currents excited therein. These large errors ordinarily constitute the chief factor that sets the low frequency limit for an array of monopoles surrounding a reflecting cylindrical screen generally of the second type of steerable antenna system described above. I

Stated in other-words, when either the sending or receiving end of a lossless transmission line is terminated in the characteristic impedance of the line, the amplitude of current delivered to the load is independent of the line length, and the time delay furnished by the line is directly proportional to its length.

Other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawings in which:

FIG. 1 is a perspective view of an exemplary antenna system according to the invention;

FIG. 2 shows a plan view of the antenna system shown in FIG. 1, additionally illustrating in block diagram form, the logical arrangement of the means for combining signals received from the individual radiator implements;

FIG. 3 is a perspective view of a preferred means for arranging a reflecting screen and counterpoise so that wind load and the required material is minimized without interfering with the desired electrical characteristics:

FIG. 4 schematically illustrates a capacitive type scanner for exchanging energy with a selected one of the individual isolation amplifiers; and

FIG. 5 schematically illustrates still another method of capacitive scanning which combines the outputs of a plurality of the individual isolation amplifiers to effectively orient the major lobe of the antenna system in a desired direction while effectively minimizing side lobes.

With reference now to the drawing, and more particularly FIG. 1 thereof, there is illustrated a perspective view of radiating elements, a reflecting screen and a counterpoise for use in a system according to the invention. The individual radiating elements 11 are vertically oriented monopoles supported above and electrically insulated from a horizontal counterpoise 12. The individual radiating elements 11 are disposed upon a circle concentric about and spaced from a cylindrical reflecting screen 13 made of conducting material.

. Referring to FIG. 2, there is shown a plan view of the antenna Structure shown in FIG. 1 together with a block diagram showing the logical arrangement of a system for providing reception from a number of selected directions.

In the illustrated exemplary system, thirty-six radiator elements are shown equiangularlyspaced, Rl-RS, respectively. To facilitate understanding the orientation of a typical installation,the view is in the azimuth plane with the radiator element R1 facing the north direction as indicated by the arrow 14.

Each of radiator elements Rl-R36 is coupled to a respective one of isolation amplifiers A1-A36 by a suitable length of transmission line diagrammatically represented. Since preferably the transmission line is beneath the counter-poise 12, the portion of the transmission lines coupling each radiator element to a respective isolation amplifierbeneath the counterpoise is represented by a broken line.

The individual isolation amplifiers are coupled to suitable combining means. When it is desired to receive signals simultaneously from a number of different directions,

the arrangement shown in FIG. 2 is advantageous. The combiners 21-28 provide signals simultaneouslyv on terminals 31-38 from eight difierently oriented directions. Each of the latter output terminals may then be connected to the input of a respective receiverto permit simultaneous reception from the dilferent directions. l-dternatively, the different output terminals may be scanned in sequence if it is desired to search in all directions for signals from distant stations seeking to establish communication. As still another alternative, the outputs of the individual isolation amplifiers may be scanned in sequence or different combinations thereof similarly scanned. However, these latter techniques will be described in greater detail below. p I

In the embodiment of FIG. 2, combiner 21 may be arranged so that the signals received by radiator elements REES-R36 and Rl-RS are combined to provide maximum receptivity to signals from the north. This maybe accomplished by introducing different delays in the lines coupling each associated isolation amplifier to the cornbiner 21 so that signals arriving from the north reach the input thereto in phase. At the same time, different degrees of attenuation may be introduced in the coupling lines so that minor lobes are suppressed.

' Each'of'the combiners functions similarly and there may be as many combiners as desired because each isoreception.

It should furnish appropriate time delays in the signals from the nine elements so that the combined signal is a maximum from that desired direction. It should control the amplitudes of the individual radiator element contributions to efiect minor lobe reduction. It should deliver the resultant signal at an impedance level suitable for use with an associated receiver.

The use of the isolation amplifiers, as shown in FIG. 2,

For transmission, a cathode follower may also be used,

but preferably with cathode coupled to the transmission line connected to the individual radiator element and the grid receiving a signal from one or more combiners. The

use of an isolation amplifier ineach radiator element feed a path permits the radiator element to contribute to a num ber of combiners, instead of just one, thereby making multiple beam operation possible. A second and no less important benefit resulting from the use of isolation amplifiers is the extension of the frequency range of the array to such an extent that one system is operable over a 6:1 band width. This is accomplished if the amplifiers are designed to have input and output impedances which closely match the associated transmission lines. Under these conditions, degeneration of the impedance match at the individual radiators does not result in large phase and amplitude errors in their currents. As stated above, such errors ordinarily constitute the chief factor which determines the low frequency limit for an array of monopoles (or dipoles) in front of a reflecting screen. However, when either the sending or receiving end of a lossless transmission line is terminated in the characteristic impedance of that line, the amplitude of current delivered to the load is independent of the line length, and the time delay introduced thereby is directly proportional to the length of the line. Specific details for designing amplifiers having such input and output impedance characteristics are well known in the art and will not be discussed further herein.

As an alternative to employing isolation amplifiers, lossy cable, such as type RG-Zl/U, may be used to feed the individual radiators from the combiners. .Whilethe losses thereby introduced may not be important in many receiving amplifications, it is generally disadvantageous in connection with transmitting installations and isolation amplifiers are generally preferred.

At the low end of the frequency range over which the antenna system operates, a backlobe will develop if the height of reflecting screen 13 is insufficient. 7 Side lobes develop at the upper end of therange where the spacing between the individual "radiator elements approaches a Wavelength. The low frequency range may be extended by raising the curtain height while the high frequency range may be extended by using more radiator elements.

The minor lobe level can be reduced further by amplitude shading; that is, reduction of excitation of the edge elements of the active quadrant over that of elements near the center. For example, if it were desired to reduce the minor lobes of the effective antenna providing signals at output terminal 31 from combiner 21, signals from the end radiator elements R33, R34, R4 and R5 would be attenuated to a greater extent than the signals received by radiator elements R36, R1 and R2. Minor lobe reduction achieved in this way results in some loss of gain due to widening the major lobe. However, it is generally possible to achieve a good compromise wherein a significant improvement in minor lobe level occurs with a very small sacrifice in major lobe gain.

The upper limit of the operating band is reached when one or more of the following conditions appear. The element spacing approaches one wavelength, causing the appearance of pronounced side lobes. The distance between radiator elements and the reflecting screen approaches one-half wavelength so that the individual radiator elements no longer radiate with sufiicient strength in the desired radial direction with"respect to the center of the array, thereby resulting in an abrupt increase in side lobes. When the radiator element height approaches one wavelength, the elevation pattern is adversely affected so that an increasing proportion of energy is directed upward at an elevation angle of about 60, presumably a useless angle for much long distance communication. This upper limit may be increased by re ducing the spacing between elements, and reducing the radiator element height.

The low frequency limit will be due to one or more of the following effects. The azimuth pattern major lobe widens as the frequency is lowered because the aperture of the anray, in terms of Wavelengths decreases. The height of the screen in wavelengths may become insufiicient to suppress the backlobe. The height of the radiators may become such a small fraction of a wavelength that too little energy is abstracted from incident radio waves, the latter factor being of minor significance. The other two may be overcome by increasing the diameter of the circle upon which the radiating elements are located while the second limitation may be overcome by increasing the height of the screen. Hence, it is possible to obtain lslatisfactory operation over an exceptionally wide band widt The directivity gain of this antenna system has a maximum value near the upper end of the frequency where the aperture in wave lengths is the greatest. The maximum effective power gain is roughly equal to the total number of radiators. This gain is based on azimuthal directivity relative to the gain of a single radiator with no reflecting curtain. Thus, effective gain may be increased by increasing the number of individual radiating elements.

Referring to FIG. 3, there is shown a preferred embodiment of the invention for operation over a wide portion of the high frequency range characterized by low wind resistance, light weight and a minimum of required material.

Only a quadrant with two radiating elements 11, a supporting base and a reflecting screen is shown in order to more clearly illustrate the structure. For transportable installations, it is desirable to use a plywood supporting base 41. The counterpoise 12 is formed of a number of horizontal wires extending radially from each radiating element 11. The reflecting curtain 13 is formed of a number of poles 42, supporting a wire 43 from which spaced conducting wires are suspended.

In the case of a monopole radiating element over a circular ground sheet functioning as a counterpoise, the latter ground sheet ought to be at least one-half wavelength at the lowest operating frequency. The use of a smaller sheet has an adverse affect on the self-impedance of the radiators. An electrically equivalent counterpoise of, for example, 32 radial wires may be employed wherein the lowest frequency of operation is typically of the order of four megacycles by utilizing number stranded aluminum wire along a radial extending 130 feet radially from the antenna. Moreover, an array of monopoles can be used effectively even though the counterpoise is overlapped as shown in FIG. 3. Preferably, insulated wire is used for the ground radial with no connection made between radials of one element and those of another where they cross. This arrangement is simple since any attempt to make connections between radials of different elements would require a very large number of such connections to be made if the array were of any appreciable size. Completely satisfactory results were obtained, in fact, using only 16 radials per element in a 32- element array. The patterns in both azimuth and elevation were nonsignificantly diiferent from those taken when a solid conducting sheet was used for the counterpoise. It was found experimentally that if the vertical wires suspended from wire 43 were separated by a distance of the order of one foot, the amplitude of the back lobe in the operating frequency range was inconsequential.

The individual radiator elements themselves may be constructed of copper wire cages of generally cylindrical form since their electrical characteristics can readily be made the same as solid sheet elements while reducing the wind load and material required.

As indicated above, the function of the combiner is to accept signals from the isolation amplifiers associated with the individual radiator elements, add the signals in the proper amplitude and phase relationships, and present the sum to the receiver at a suitable impedance level.

One type of combiner is shown in FIG. 2 and was described above. It included'a set of cables of appropriate lengths which connect in parallel the signals from nine adjacent radiator elements. The cable lengths and element positions so correspond as to make each such parallel junction effectively the proper terminal of a directional receiving antenna. In the arrangement illustrated in FIG. 1, the beam orientations are spaced at intervals of 45 in azimuth. A typical value of cathode follower output impedance is ohms, which matches available cable of the same characteristic impedance. Nine such cables cou pled from respective isolation amplifiers would have an impedance of approximately 14 ohms when connected in parallel. It may be desirable to step up this impedance by means of a transformer or other impedance transformation device. Amplitude shading for minor lobe control may be effected by inserting attenuating pads in the cables coupled to radiators near the edge of the active quadrant Referring to FIG. 4, there is illustrated a schematic representation of a capacitive type scanner for selectively coupling the output of each isolation amplifier to a single receiver or transmitter. For each isolation amplifier there is a stator plate, such as plate 41' connected to a terminal 42 which in turn is electrically connected to a respective isolation amplifier. For example, terminal 42' would be connected to isolation amplifier A1 in FIG. 2. A rotor plate 43 is pivotally mounted to rotate about the axis of the circle defined by the stator plates. The rotor plate 43' is electrically connected to terminal 44 which in turn is coupled to the receiver or transmitter, depending upon whether transmission or reception is desired. The means for rotating the rotor element 43 are not shown. Such rotation may be accomplished manually or by a suitable electric motor.

Referring to FIG. 5, there is shown still another type of-scanner in which the stator is the same as that Shown in FIG. 4, but the rotor includes a pair of symmetrically arranged transmission lines 45 and 46 capacitively coupled to a number of stator plates to couple signals between terminal 44 and stator plates opposite the rotor. The transmission lines 45 and 46 may be so dimensioned thatappropriate phase delays are imparted to each received signal to narrow the major lobe while imparting increased attenuation to signals received from end elements so that minor lobe reduction is effected. The desired phasing may be effected because received signals from the center stator in the selected quadrant must travel over a greater distance in the transmission lines 45 and 46, which may, for example, be parallel plate transmission lines with the rotor forming one plate and a top covering plate (not shown so as not to obscure the principles of the scanner) forming the other. Variations in attenuation may be effected by increasing the spacing between the edge stator plates 47 and 48 and the transmission lines 46 and 45, respectively. This scanner is especially advantageous since it provides the desired phase shift, attenuation, signal combination and continuous scanning. For a detailed discussion of a scanner functioning in this manner, reference is made to the copending application of Lindsay Russell entitled Phasing System, Serial No. 766,643, filed October 8,

There has been described a novel directional antenna system which incorporates numerous features heretofore unavailable in a single system operable over such a wide bandwidth. While the system has been described with specific reference to long-distance short-wave communication systems, many of the properties are advantageous in connection with VHF, UHF, microwave and lower frequency systems.

As an example of a suitable unit for operating over a wide high frequency range, suitable dimensions for the diameter of the array shown partially in FIG. 3 are as follows:

Diameter of curtain 13334 feet.

Diameter of the circle upon which radiator elements 11 are located369 feet.

Outside diameter of counterpoise 12-669 feet.

Height of radiator elements 1125 feet.

Height of poles 42-65 feet.

g 9 Minimum height of cable 43-60 feet; Screen wire diameter-0.187 inch. Counterpoise wire diameter-0.475 inch.

The specific structures, dimensions and combiners described herein are by way of example for illustrating the best mode now contemplated for practicing the invention. It is apparent that those skilled in the art may now make numerous modifications of and departures from the specific techniques described herein without departing from the inventive concepts. Consequently, the invention is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

1. An antenna system comprising, a plurality of individual radiating elements disposed upon a curve spaced from a reflecting screen, combining means, a like plurality of coupling means connected between a respective one of said radiating elements and said combining means to electrically isolate each radiating element from the others, utilization apparatus, and means for selectively coupling said utilization apparatus through said com bining means to a selected group of said radiating elements, the number in said selected group being less than said plurality.

2. An antenna system in accordance with claim 1 and further comprising, delay means for furnishing a selected degree of 'time delay diiference in the signal paths from said utilization apparatus to those of said radiating elements in said selected group to orient the major lobe of the directivity pattern of said antenna system in a prescribed direction related to the group selected. I

3. An antenna system in accordance with claim 1 and further comprising, attenuating means for furnishing a selected degree of attenuation difference in the signal paths from said utilization apparatus to those of said radiating elements in said selected group to suppress minor lobes of the directivity pattern of said antenna system.

4. An antenna system in accordance with claim 1 and further comprising, delay means for furnishing a selected degree of time delay difference in the signal paths from said utilization apparatus to those of said radiating elements in said selected group to orient the major lobe of the directivity pattern of said antenna system in a prescribed direction related to. the group selected, and attenuating means for furnishing a selected degree of attenuation difference in said signal paths to suppress minor lobes of the ,directivity pattern of said system.

5. An antenna system in accordance with claim 1 wherein said coupling means includes an isolation amplifier having two terminal pairs, one terminal pair being coupled by a transmission line to an associated radiating element, the other terminal pair being coupled to said combining means, the impedance presented by said isolation amplifier at said one terminal pair being substantially equal to the characteristic impedance of said transmission line.

6. An antenna system in accordance with claim 5 wherein said other terminal pair is coupled to said combining means by a second transmission line terminated in its characteristic impedance.

7. An antenna system in accordance with claim 1 g 10 radiating elements, said group comprising consecutively arranged ones of said radiating elements, the number in said group being less than said plurality, the location of said selected group on said curve generally facing a direction corresponding to the selected orientation of the main lobe of the directivity pattern of said antenna system in a plane orthogonal to said axis, means for coupling each element in said group to said combining means, said coupling means including means for furnishing diiferent delay intervals between said combining means and each element to cause the angle of said major lobe to be relatively narrow, and means for attenuating energy exchanged with at least one of said elements differently from that exchanged with the others to reduce the magnitude of minor lobes.

10. A directional antenna system in accordance with claim 9 and further comprising a counterpoise beneath each of said monopole radiating elements.

11. A directional antenna system in accordance with claim 10 wherein said reflecting cylinder comprises a plurality of supporting poles, a cable supported by said poles, and spaced conducting wires suspended from said cable.

12. A directional antenna system in accordance with claim 10 wherein each counterpoise comprises a plurality of radially arranged conducting wires emanating from the axis of an associated monopole radiating element.

13. A directional antenna system in accordance with claim 4 wherein said delay means and attenuating means comprises, a plurality of spaced plates, each coupled to a respective one of said individual radiating elements, electromagnetic energy transmission means coupled to a group of consecutive ones of said spaced plates in spaced relationship therewith and furnishing different path lengths therethrough for energy passing between each spaced plate of said group and said utilization apparatus,

the spacing between said transmission means and a spaced plate being related to the degree of attenuation imparted to energy transmitted through the latter plate, and means for selectively controlling the relative orientation between said spaced plates and said transmission means to determine said selected group of radiating elements.

14. A directional antenna system operative over a wide frequency range between first and second radio frequencies comprising, means defining a ground plane, a plurality of similar monopole radiating elements perpendicular to and on one side of said ground plane and disposed upon a circle, a reflecting conducting cylindrical screen inside of said circle andconcentric about the axis of said circle extending from said ground plane on said one side thereof, the axial length of said screen being greater than the length of said monopole radiating elements, the separation between adjacent monopoles being less than a wavelength of energy at said second radio frequency, the radial distance between said circle and said screen being less than half the latter wavelength, said screen axial length being sufficiently high to suppress back lobes at said first frequency, the length of each monopole being sufiicient to intercept a significant amount of energy at a said first frequency, combining means for exchanging high frequency energy with a selected group of said radiating elements, said group comprising consecutively arranged ones of said radiating elements, the number in said group being less than said plurality, the location of said selected group on said circle generally facing a direction corresponding to the selected orientation of the main lobe of the directivity pattern of said antenna system in a plane orthogonal to said axis within said frequency range, and a plurality of coupling means each connected between a respective one of said radiating elements and said combining means to electrically isolate each radiating element from the others.

15. Apparatus in accordance with claim 13 wherein said spaced plates are circumferentially disposed upon a circle and said electromagnetic energy transmission means comprises, means defining a pair of wave transmission 11 channels eachhaving a first portion extending radially Outward from the center of said circle joining a second portion inside, adjacent and generally parallel to said said spaced plates and said electromagnetieenergy transmission means.

References Cited in the" file of this patent UNITED STATES PATENTS Kramar Feb. 13, 1940 Busignies 11113} 6', 1948 Bagnall Apr. 5, 1949 

